1. Field of the Invention
This invention relates to twin extruders, and more particularly to increasing the processing capacity of a twin extruder which melts and kneads powdery resin material, or a mixture of powdery or granular resin and powdery material.
2. Related Art
A conventional twin extruder has a material supplying section, a conveying section, a kneading section, a degassing section, and a discharging section with two screws rotatably engaged with each other in the internal hole of a cylinder, In the twin extruder, resin material is supplied through the material supplying inlet of the material supplying section into the cylinder under the condition that the temperature of the cylinder is externally controlled to a predetermined value and the screws are kept turned in the cylinder. The resin material thus supplied is molten by the heat of the cylinder and by the friction of the resin materials which is produced by the kneading action of the screws in the cylinder while passing through the conveying section, and then kneaded at the kneading section, and then moved to the degassing section, where gas such as volatile gas is removed from the resin materials. The resin material thus processed is moved to the discharging section, where it is subjected to discharging pressure, so that it is pushed out of the cylinder.
If more than several tens of percent of the material to be kneaded is powdery material 10 microns or less in average grain size, or the material to be kneaded is all powdery material 10 microns or less in average grain size, then gas (air) held in the powdery material (by the particles) is not readily separated therefrom.
In an ordinary twin extruder, a material to be kneaded (hereinafter referred to as "a kneading material", when applicable) is conveyed and molten while holding gas. Hence, the gas held in the kneading material caught between the wall of the internal hole of the cylinder and the grooves of the screws, is expanded as the temperature of the kneading material increases, thus forming cavities in the kneading material being held therein. It is difficult for the gas to flow towards the downstream end of the cylinder because the downstream end portion of the cylinder is filled with the kneading material which has been molten. Hence, the gas thus held is caused to intermittently or instantaneously flow back towards the material supplying section where the gas can relatively readily flow because the kneading material therein is deformable because it is not molten yet.
As a result, the amount of supply of the kneading material changes unstably, and therefore it becomes difficult for the twin extruder to perform its operation stably. That is, the extruder is lowered in the capacity of processing a kneading material. And it is difficult to increase the processing capacity and the processing efficiency of the extruder.
With respect to the discharging of gas from a kneading material, Unexamined Japanese Utility Patent Publication No. 17321/1990, 76023/1990 and 90732/1982 have proposed the following twin extruders:
In case of the twin extruder disclosed by the Unexamined Japanese Utility Patent Publication No. 17321/1990, in the material supplying section and the conveying section, the internal hole of the cylinder is polygonal in cross section.
In the extruder, a kneading material conveyed through the material supplying section to the conveying section is heated, compressed, and molten. During this operation, the gas is separated from the kneading material, and is moved back into the material supplying section through the gaps between the wall of the cylinder's internal hole polygonal in cross section and the outer rotational peripheries of the screws. The gas thus moved back is discharged through the material supplying inlet out of the cylinder.
In case of the twin extruder of the Unexamined Japanese Utility Model Publication No. 76023/1990, in the material supplying section and the conveying section, a porous member is used to provide the internal hole for the cylinder in such a manner that a gas discharging space is formed between the outer surface of the porous member and the cylinder.
In the material supplying section and the conveying section, the gas separated from the kneading material passes through the pores of the porous member, thus being discharged through the gas discharging space out of the cylinder similarly as in the above-described case.
In the case of the twin extruder of the Japanese Utility Model Publication No. 90731/1982, in the material supplying section and the conveying section the internal hole of the cylinder has a groove in its upper wall which is extended axially, and a gas discharging vent hole is provided at the end of the groove, just behind the material supplying inlet of the material supplying section.
Similarly as in the above-described cases, in the material supplying section and the conveying section the gas separated from the kneading material passes through the groove and is then discharged through the vent hole out of the cylinder.
In almost all the conventional twin extruders proposed in the art including the above-described conventional ones, a gas flow path is formed which allows the gas separated from the kneading material to flow back towards the material supplying section, so that the gas is continuously and smoothly discharged out of the cylinder. This contributes to improvement of the twin extruder in safety operation and in processing capacity.
However, it should be noted that the flow path formed inside the cylinder is a part of the cylinder's internal hole, and the gas flowing back along the flow path is moved while contacting part of the kneading material, so that it flows back while drawing the powdery material; that is, the gas is moved together with the powdery material thus drawn.
Hence, the gas discharging section adapted to discharge the gas thus separated needs means for removing and receiving the powdery material from the gas.
In the conveying section, the kneading material is molten; that is, its phase is changed from solid phase to liquid phase.
The kneading material of solid phase is high in fluidity. Hence, the kneading material of powdery and granular particles will fill the grooves formed in the cylinder's internal hole with ease. In addition, particularly in the case where the kneading material is organic powdery material, the kneading material, being brought into contact with the cylinder heated, is partially molten and stuck to the grooves, thus gradually filling the grooves, which may block the flow of the gas.
On the other hand, whether the kneading material is powdery or granular or no matter how it is molten, the kneading material is liable to be settled at the corners of the cylinder's internal hole which correspond to the vertexes of the polygonal cross-section of the latter, or the bottoms of the grooves which are formed in the cylinder's internal hole in such a manner that they are extended axially. The kneading material thus settled may be stuck there.
In the case where, in the material supplying section and the conveying section, the internal hole of the cylinder is made of the porous material, the pores of the porous material may be gradually clogged up with the powdery material. In this case, it may become impossible to discharge the gas separated from the kneading material.